In 1954, a typhoon sank five ferry boats in Japan's Tsugaru Strait and killed 1,430 people. In response to public outrage, the Japanese government searched for a safer way to cross the dangerous strait. With such unpredictable weather conditions, engineers agreed that a bridge would be too risky to build. A tunnel seemed a perfect solution. Ten years later, work began on what would be the longest and hardest underwater dig ever attempted.

Engineers couldn't use a tunnel boring machine to carve the Seikan Tunnel because the rock and soil beneath the Tsugaru Strait was random and unpredictable. Instead, tunnel workers painstakingly drilled and blasted 33 miles through a major earthquake zone to link the main Japanese island of Honshu with the northern island of Hokkaido. Today, the Seikan Tunnel is the longest railroad tunnel in the world at 33.4 miles in length, 14.3 miles of which lie under the Tsugaru Strait.

Three stories high and 800 feet below the sea, the main tunnel was designed to serve the Shinkansen, Japan's high-speed bullet train. Unfortunately, the cost of extending the Shinkansen service through the new tunnel proved to be too expensive. In fact, air travel today between Honshu and Hokkaido is quicker and almost as cheap as rail travel through the tunnel. Despite its limited use, the Seikan Tunnel remains one of the greatest engineering feats of the 20th century.

Here's how this tunnel stacks up against some of the longest tunnels in the world.
(total length, in feet)

Seikan Tunnel174,240' (33 miles)

Fast Facts:

More than 2,800 tons of explosives were used in the construction of the tunnel.

One hundred sixty-eight thousand tons of steel was used in the construction of the tunnel. That's enough steel to build four Petronas Towers!

The railway track runs 787 feet below the surface of the sea, making it the deepest railway line in the world.

During construction in 1976, tunnel workers hit a patch of soft rock with disastrous results. Water gushed into the tunnel at a whopping rate of 80 tons per minute. It took more than two months to control the flood. Luckily, no lives were lost.